1. Field of the Invention
The present invention relates generally to thin film magnetic structures intended for giant magneto-resistance (GMR) applications and more particularly to such devices including crystalline layers of one or more nonferromagnetic metals alternating with crystalline layers of one or more ferromagnetic metals having closely matched crystalline symmetry.
2. Description of the Background Art
In this description and in the claims that follows, the terms "metal" includes alloys (crystalline and amorphous) and metallic elements.
Using thin film deposition technologies such as magnetron sputtering, a family of thin film structures has recently been developed in which the material resistance is a sensitive function of applied magnetic fields, so called giant magnetoresistance, GMR. Typically, GMR structures include alternating magnetically hard and magnetically soft layers (relative to each other) of 3d transition metals, all having closely matched (i.e., lattice spacings within 2% of each other) face-centered cubic (fcc) symmetry, spaced from each other by intervening layers of Cu (also having fcc symmetry closely matched with the fcc symmetry of the magnetic layers). The magnetically hard and soft layers may be formed of materials which are magnetically hard or magnetically soft compared to each other, or they may be formed of the same materials, with their relative magnetic coercivities being determined by layer thickness. These structures are being researched and optimized for use in a number of applications in a large number of university, government and industrial laboratories. In many applications, the physical vapor deposition of those structures, e.g., by magnetron or ion beam sputtering, results in the growth of large crystal grains. This large grain growth may also occur even if a single magnetic layer is grown on a thick layer of Cu, such as may be used to make electrical connections to the structures. In contrast to the case where grain sizes are less than about 10 nm, the presence of larger grains allows the magnetic materials to manifest their magneto-crystalline anisotropy. This magneto-crystalline anisotropy produces a strong tendency for the magnetic moments to align along local crystallographic directions and hence become less sensitive to the external magnetic fields to which the materials are intended to respond.
Using materials such as Co, NiFe alloys (e.g., Permalloy.TM.), CoFe alloys (usually about 5 to 97 atomic percent Co) and NiCoFe alloys as ferromagnetic layers and with Cu as a spacer between them, structures have been made for which the resistivity can change by up to about 1% per Oe of applied field, with total resistance changes up to 25% (in trilayers; somewhat higher in multiple stacks) for current flow in the film plane, CIP, and even larger values have been reported for current flow perpendicular to the film plane, CPP, for multilayer stacks.
The resistance change is governed mainly by the so-called "spin-valve" effect in which a part of the resistance changes proportionally to the cosine of the angle between the magnetic moments of adjacent layers. Therefore, achievement of a large resistance change requires that the external applied field change the angles of the magnetic moments of some layers with respect to their neighbors from antiparallel to parallel. While very large resistance changes have been realized in multilayer stacks with combinations such as Fe/Cr, Co/Cu, and Fe/Ru, especially, as layer pairs, very large fields are required to realize the maximum effects, so that the percent change per Oe is far below 1 percent. These materials therefore are useful mainly as sensors for large magnetic fields.
As the thickness of the spacer layer, most commonly Cu, is increased in the range of less than 1 to 5 nm, the largest source of this opposition to magnetic alignment, the antiferromagnetic exchange interactions, disappears rapidly, and much smaller fields are required to achieve the magnetic switching, although the size of the GMR effect also decreases somewhat. However, an additional mechanism must often be introduced to achieve the antiparallel high resistance state in low applied fields. If the resistance change for an application can be less than or about 10%, this alignment is often produced by introducing a separate "exchange bias" layer adjacent to one of two magnetic layers. Another method of achieving an antiparallel alignment applicable to multilayer stacks is to introduce Co into half of the layers. This introduction of Co increases the coercivity of that layer relative to compositions with little or no Co, allowing a low field to reverse the low Co layer without affecting the Co rich layer.
Unfortunately, in cases where the thickness of a multilayer GMR stack exceeds about 10 nm, or in other cases where it is desirable to grow the stack onto a pre-deposited thick Cu "buss bar" to provide electrical connection to the magneto-resistor and to provide magnetic switching if needed, an additional complication arises. Because the crystalline grains of the alloys used grow in size rapidly with thickness for useful GMR systems, and this growth continues through the stack, the surfaces become rough and grain sizes rapidly exceed 100 .ANG.. A number of undesirable consequences follow from this mode of growth. The physical roughness increases, which introduces a source of coupling between adjacent layers, commonly referred to as Neel or orange peel coupling. This coupling makes it harder to achieve the antiparallel state in low fields. The roughness also affects and randomizes the details of the magnetization process, especially if the process proceeds by motion of a domain wall. Additionally, this roughness may complicate further processing. Although the physical roughness can be removed by polishing, the large crystal faces can still replicate their crystalline orientation onto the magnetic overlayers, with highly adverse consequences on the magnetic properties and switching behavior of the overlayers. These adverse consequences result mainly from the enhanced magnetocrystalline anisotropy, which has cubic symmetry.
Except for a few carefully selected alloys, the cubic anisotropies in the Fe-Ni-Co ternary system are considerable, having effective anisotropy fields on the order of 1000 Oe in Co and Fe rich alloys. In this system, there is likely to be at best a few lines, in a ternary diagram of compositions for which this anisotropy characterized by an energy/volume, K.sub.1, is near zero. Because zero or minimal crystalline anisotropy is desirable for GMR uses, only a few compositions are optimal for GMR use. Hence, the presence of large grains in GMR layers can prevent attainment of useful properties. Fortunately, the composition known as Permalloy.TM., Py, which is usually given as 80% Ni and 20% Fe, has close to ideal properties in that the isotropic magnetostriction is zero and K.sub.1 is small (2K.sub.1 /M.sub.s.about.5 Oe). Permalloy.TM. also has a low value of growth-induced magnetic anisotropy, .about.3-4 Oe, which allows it to switch in a few Oe regardless of grain size. However, Py gives only about 1/2 the resistance change, when using in GMR structures, compared to cobalt-rich alloys. Therefore, Py alloys incorporating slight to large amounts of Co have been explored to take advantage of two desirable properties. The first is larger GMR effects compared to Py without Co. The second desirable property is more subtle: briefly, the large value of growth-induced anisotropy, up to 20 Oe, in fine grained Co rich alloys can produce a coercivity of this size. This coercivity allows a multilayer stack using this material in layers alternating with Py and Cu to be put in its high resistance state by field cycling appropriate to magnetic memory designs.
Unfortunately, in multilayer stacks, because of the large grain size, the high cubic anisotropy of the Co-rich films deleteriously affect GMR performance. Consequently, the resistance changes as a function of applied fields in materials made of these alloys, as compared to Py-based systems, or thin, fine-grained CoFe alloy films, become very broad with the changes extending to much larger fields and the switching becoming rounded. Furthermore, the desired antiparallel state having the highest resistance becomes unattainable at low fields. Also, a very small device, such as a memory element having a magnetic multilayer thickness of about 100 nm, would incorporate only a few crystalline grains. The size, shape and orientation of these few crystalline grains would be highly random. As a result, there would be large variations in magnetic properties from element to element.